Till the 1970s, in the hub bearing for use in a car, the design of arranging two standard bearings conforming to ISO specification was mainly adopted. In the 1980s, to provide improvement in the mounting of the bearing on the car, car makers came to adopt a double row angular contact ball bearing or a double row tapered roller bearing in which outer rings of bearings arranged were held together as a unit with the backs of the bearings opposed to each other. These hub bearings are called the first generation hub bearing (GEN 1). By holding the outer rings together as a unit, an initial axial gap was set to a proper value in assembling the hub bearing. Thus it was unnecessary to adjust a preload in mounting the hub bearing on the car. Thereafter a double row bearing, having a flange portion provided on the outer ring, which is called the second generation hub bearing (GEN 2) was developed. Because merely the arrangement of two standard bearings had a limitation in a decrease of the weight and size of the hub bearing, a shaft (hub ring) and a housing (knuckle) which are peripheral parts of the bearing were held together as a unit to decrease the number of parts and the weight of the hub bearing. By changing the fixing of the hub bearing to the knuckle from press fit to bolt tightening, it became easy to mount the hub bearing on the car. In the third generation hub bearing (GEN 3), the shaft (hub ring) and the inner ring of the bearing were held together as a unit to remove excess thickness, and the line-assembling efficiency was improved. Recently the fourth generation hub joint (GEN 4) in which the hub bearing and a constant velocity joint are held together as a unit has been developed.
Recently the GEN 2 and the GEN 3 are increasingly adopted because as described above, in the GEN 2 and the GEN 3, the workability in mounting the bearing on the car is improved considerably, stability in travel of the car is improved owing to a decreases in the amount of a spring load caused by the elimination of press fit, and the use of a knuckle made of a light alloy effective for decreasing fuel expenses can be adopted.
Paying attention to the material of the bearing, till the adoption of the GEN 1, ordinary bearing steel (for example, SUJ2) was used. In the GEN 2 and the GEN 3 having the flange mounted on the outer ring, the carbon steel for machine structural use such as S53C which has a favorable forgeability and is inexpensive came to be used. The carbon steel for machine structural use secures the rolling fatigue strength of the bearing portion by heat-treating the raceway portion with a high-frequency wave. But the carbon steel for machine structural use has a low surface strength because it contains a small amount of an alloy component and is inferior to the bearing steel in the resistance of the bearing to peeling starting from the surface thereof. Therefore in the same lubricating specification as that for the GEN 1, the carbon steel for machine structural use used in the GEN2 and the GEN3 has an inferior durability when use conditions therefor are strict.
Regarding the improvement of the grease for the hub bearing, the decrease of the rotational torque by the adoption of a low-viscosity base oil (see patent document 1) and the impartation of conductivity for the removal of static electricity (see patent document 2) are known. But since the adoption of the GEN2, no progress has been made on the improvement of the resistance of the hub bearing to the peeling starting from the surface thereof. Because the weights of parts are decreased to reduce fuel expense and improve operation stability, the size of the hub bearing is necessarily decreased, and owing to a rise of a face pressure, the lubricating condition has become strict.
There is a possibility that a tire-driving method using an in-wheel motor is adopted in the future. In that case, it is forecast that the temperature of the hub bearing will rise and the lubricating condition will be strict.
Patent document 1: Japanese Patent Application Laid-Open No. 2003-239999
Patent document 2: Japanese Patent Application Laid-Open No. 2004-169862